The present invention relates generally to a process for the substantially non-dehydration drying of solids-containing aqueous systems, and particularly to the use of such a process in the preparation of free-flowing, non-hygroscopic powders from such aqueous systems.
The prior art is replete with discussions of the problems inherent in dealing with products produced and utilized in association with any significant degree of liquid. Thus, for example, solids-containing aqueous systems such as milk, natural fruit and vegetable juices, soft drinks, sauces, puddings, extracts of coffee and tea, and the like present numerous difficulties to manufacturers, distributors and consumers in the packaging, handling, shipping, and storage requirements imposed by virtue of the water content of these products.
In response to these problems, the prior art has proposed a variety of methods to accomplish the partial or substantially complete removal of water from such products and thereby produce reduced volume products, ideally substantially water-free, flowable powders, which may be reconstituted, upon addition of water, to the dilution desired for ultimate use. Conventional processes for accomplishing this dehydration of solidscontaining aqueous systems. are based upon the removal of water in its vapor state, for example, by its evaporation from a liquid system or its sublimation from a frozen system.
Although the removal of water from aqueous systems according to conventional processes presents in the abstract few, if any, technical problems, it has long been recognized that the conditions under which such removal of water is accomplished may deleteriously affect the product such that, upon reconstitution, the product is degraded or otherwise dissimilar as compared to the starting product. Thus, for example, while heating a solids-containing aqueous product to boiling is an effective means of removing water per se, the prolonged heating may substantially alter the product in an undesirable manner.
The foregoing problem is particularly attenuated where the product being dehydrated is a foodstuff having desirably retained organoleptic properties, such as flavor, color, aroma, mouthfeel, texture or appearance. To take but one example, dehydration of natural fruit juices, such as orange juice, has been practiced by spray drying, vacuum shelf or belt drying, drum drying, foam-mat drying and freeze drying. These processes generally involve the concentration of juice followed by the dehydration of the concentrate through the application of heat and/or vacuum under controlled conditions. Many difficulties have been encountered in such processes, particularly those processes yielding dry, solid products, and heretofore there has been developed no practical and economical process for the production of free-flowing juice powders that can be readily reconstituted by the addition of water to yield a product closely resembling a fresh juice beverage in quality, flavor and appearance.
Spray drying is not satisfactory in many cases because the dried product is usually found to be inferior to the original juice concentrate in flavor and stability. Such products are extremely hygroscopic, due to the natural hygroscopicity of juice solids. In many cases, it is difficult to spray dry the juices without the addition of large amounts of carriers. If used, these carriers can impart an undesirable taste to the product and are generally unacceptable at the concentration levels that are required for satisfactory spray drying.
Vacuum shelf or belt drying also imparts a cooked flavor to the dried product and frequently causes browning of the product. This results in an undesirable taste and detracts from visual acceptance. Moreover, because of the high vacuum conditions of these drying processes, desirable volatiles are lost through vaporization.
In drum drying, the sugar present in the juice prevents the formation of a sheet which can be easily removed by the dryer's doctor blade or scraper. Instead of being able to remove a continuous dried sheet from the dryer, the dried juice yields a gummy mass after heating which collects at the doctor blade and disrupts the drying operation. This process also imparts an undesirable off-flavor to the product.
Foam-mat drying involves the use of a foaming agent in the juice during drying. This foaming agent causes formation of extremely small, microscopic bubbles and produces a larger surface for rapid and more complete drying of the juice powders. However, this process is unsatisfactory since the powders produced often contain incorporated microscopic air bubbles which can promote browning and thus may be detrimental to the flavor and stability of the powder. In addition, "air sols" or colloidal dispersions of these air bubbles are formed throughout the solution upon reconstitution of the dried product. These bubbles impart to the reconstituted juice an unnatural, white, cloudy or milky appearance. After the reconstituted product stands a very short time, the "air sols" rise and collect on the surface as a dense layer of foam. The presence of this milky appearance and particularly the presence of the foam are detrimental to the physical appearance of the products and adversely affect their commercial acceptance.
Freeze drying in general produces the most acceptable product of conventional drying processes. However, the slow rate of sublimation from the frozen state and the high vacuum associated with the process result in loss of desirable volatiles through vaporization and make the process relatively expensive. Also, the dried product produced is hygroscopic and this adversely affects the storage stability and spoonability of the dried product.
U.S. Pat. No. 3,619,294 (Black) discloses a method of drying in which the liquid to be dried is absorbed by granules having an internal capillary network. After the granules are impregnated, the granules are dried in ordinary drying equipment for granular materials, such as a gas fired rotary dryer. One big disadvantage of that system is that it requires the spraying of the liquid onto a bed of granules, and the rate of spraying must be controlled to prevent agglomeration. Thus successive impregnation cycles are required. After each impregnation cycle the granules are usually dried by the application of heat.
A significant improvement in the art of preparing free-flowing, non-hygroscopic food and/or average powders is described in U.S. Pat. No. 3,953,615 to Gupta and Shillinglaw. According to the "hydration drying" method described in that patent, juice concentrate, coffee extracts or tea extracts are admixed with a suitable amount of anhydrous dextrose such that most of the water present in the concentrate is converted into a "bound" form, i.e., by formation, with the anhydrous dextrose, of dextrose monohydrate. In this manner, the Gupta, et al. patent teaches that the water present in the concentrate is no longer readily available for wetting or subsequent interaction with other particles, and the concentrate is reduced to a free-flowing, non-hygroscopic powder. The Gupta, et al patent further teaches that the rate at which this drying, i.e., hydration of the dextrose, takes place can be expedited by subjecting the concentrate/anhydrous dextrose mixture to temperatures in the range of 35.degree. C. to 85.degree. C. By virtue of the fact that the concentrate is not subjected to harsh water removal conditions, products prepared by reconstitution of the powders in water are stated to closely resemble those prepared from the starting concentrate.
The Gupta, et al. patent thus describes a useful method for reducing tea, coffee and juice concentrates to free-flowing, non-hygroscopic powders, and is useful in preparing products compatible with the sweetness and bulking imparted by the overt addition of anhydrous dextrose. We have found, however, that additional refinements in this method result in significant improvement in both the rate of moisture equilibration (and, hence, drying) and the quality of the products produced. Specifically, Gupta and Shillinglaw specify a temperature range which has been found to be disadvantageous. For example, if temperatures over 50.degree. C. are encountered the hydration drying process is adversely affected. Additionally, the Gupta et al. reference is directed to a process that utilizes low agitating speeds. Applicant has found that violent agitation is significantly more efficient so long as temperatures are maintained at or below 50.degree. C.
U.S. Pat. No. 2,147,238 (Bruce) discloses a process for reducing fruit juices to a solid state. The process described entails dissolving dextrose hydrate in a fruit juice and heating the solution to between 120.degree. to 150.degree. Fahrenheit. The solution is then cooled and seeded with dry dextrose. The Bruce reference fails to differentiate between the alpha and beta anomeric conformation ring structure of dextrose. The use of anhydrous beta dextrose has been found to reduce the hydration drying capacity of the method of the present invention.
Other patents in the art of drying liquids include U.S. Pat. Nos. 3,975,547 (D'Ercole): 1,481,893 (Fleming); 2,367,131 (Leo); 3,989,852 (Palmer); 2,826,504; 2,567,038 (Stevens); 2,513,813 (Milleville); 2,906,630 (Turkot); 2,970,978 (Stevens); and 2,471,678 (Flosdorf).
There are also a number of foreign patents relating in general to the art of drying, these are Japanese Patent application Publication Nos. 7814/66; 25109/69; 20933/70; 349/66; Netherland Pat. No. 277729; and German Pat. No. 716723.